Originally Posted by
DaviddesJ
Well, that's the opposite of what was said above, that it draws power from the plane's main systems when available, and switches to battery when airplane power is not available. I freely admit I don't know. If there is no possible difference in the environment on the 787 from other planes, then there is no difference. But I'd look for commonalities before just assuming it's a huge coincidence.
The wiring to the cockpit is to test or arm if the option is fitted. That's the limit of my knowledge there. Quite how power distribution is in that regard I'm really not sure. It could simply be a case of making an internal circuit with an external switch.
I am not necessarily assuming coincidence, I merely stated that it "seems cruel luck" rather than is cruel luck.